It used to be called the 'year off' between school and university, and you were lucky if you got to France. Now it's a gap year - essential for the enterprising sixth-former - and it doesn't count unless you go somewhere very Third World. The gap-year phenomenon originated with the months left over to Oxbridge applicants between entrance exams in November and the start of the next academic year. Nowadays, Oxford and Cambridge make conditional offers before A-levels, like everyone else, but the gap year has exploded, becoming a kind of finishing school for the middle classes. One estimate puts the numbers taking such a break at between 100,000 and 200,000 a year.

The notion that digging a well in Ethiopia is a valuable experience has been helped by backing from the Department for Education and Employment and Ucas, the university admissions service, both of which have endorsed the value of deferring entry to university. The internet has, meanwhile, given pupils greater access to organisations providing gap-year experiences.

As a result, the number of gap-year providers has mushroomed into the hundreds. They offer expeditions, volunteering, work placements and cultural and language courses. Some are charities, others highly profitable companies. It is difficult to tell them apart at first sight - any sort of organisation can promise that you, like Prince William, can scrub floors in Chile - but they turn out to be very different.

Lisa O'Donnelly applied to teach English in Ethiopia with a commercial provider. Unusually, she had more than one year off before starting at the School of Oriental and African Studies in London and already had a qualification in teaching English as a foreign language. When she arrived in Africa, she found there was no school and no one to teach. 'I was in a physiotherapy centre for children with polio from all over Ethiopia. It was a good project, but there was nothing there for me to do. In England, I was told I needed an induction, for which I was charged £250. It was mostly about teaching English and I didn't want to go, because I'd already taught for a year at this stage, in India and Nepal.

'There were two of us going to this project in Ethiopia, but a month beforehand the other girl dropped out. I was warned if I didn't go, I'd lose all my money. I was also told there would be a language induction in the first week, but the local representative put me in a hotel five miles outside Addis and then disappeared for seven days. When I complained, I was told I didn't understand the Third World.

'The company claimed to have checked out the project, but it turned out that no one had been there. I paid £250 a month for a room on the project compound, even though my hotel had cost about 50p a night. I asked for a breakdown of the costs from the company, but they couldn't give me one. They said everything was admin.'

Last week, O'Donnelly, who fought to get back the several thousand pounds she paid, spoke at a conference in Manchester organised by Tourism Concern aiming to bring together students and provider organisations, and to consider whether gap years are as good for dirt-poor destinations as for the young Brits who descend on them.

Michael Lomotey, who is half-English, half-Ghanaian, organised the conference after being shocked by the behaviour of British gap-year students in Ghana. 'Their cultural insensitivity is incredible. They dress inappropriately, wearing shorts in the marketplace and they haggle without smiling. I've seen British gappers get into a fist fight with a taxi driver over the equivalent of 20p, which would matter to him, but not to them.

'On one flight out to Ghana, I travelled with 70 gappers. Most had received no training, no induction into the culture. I kept wondering if 10 of them were going to build a well and they'd each paid £1,000, what could the community have done with that money?'

Last May, 20 of the larger gap providers formed the Year Out Group, a trade association, in a bid to prevent the kind of experiences that Lisa had. The group now has 23 members and, according to its chief executive Richard Oliver, 'aims to agree standards and promote the benefits of a structured year out'. Its website offers a useful list of questions for students and parents: what exactly will I be doing? Is the host organisation paid? What are the hidden costs? And how will I be supported?

As Lavinia Bristol, who runs Project Trust, one of the best gap-year organisations, says: 'Eighteen-year-olds are extraordinarily bad at being discerning consumers. Even on our selection courses, they don't ask questions that you'd think might be obvious, such as how they're going to be looked after or how the project's been vetted.'

But more radical critics, like Jim Cogan, founder of Student Partnerships Worldwide, one of the oldest and most distinguished organisations, question whether gap years in poor countries are of any benefit. 'The danger is that if untrained British students go out to poor countries that are riddled with disease and malnutrition, they are likely to end up debauching themselves by the ocean or on the shores of Lake Malawi in little hedonistic enclaves. They can enjoy the cheap cost of living, have an interesting four months and come home. There is a big moral issue here, which is conveniently being overlooked.'

The trouble is that there are many different kinds of gap-year provider, some with more honourable intentions than others. On the one hand, there are organisa tions such as Project Trust, a charity that took over Voluntary Service Overseas' 18-year-olds programme, and selects its 200 participants on the basis of a five-day assessment on the Isle of Coll in Scotland. It provides a week-long training course and sends gappers to their projects for an entire year. Student Partnerships Worldwide trains 250 British students alongside the same number of young people from host countries for a month and specialises in teaching adolescents in countries with high levels of Aids and HIV how to avoid infection.

On the other hand, Teaching and Projects Abroad is a commercial company. Its 1,000 students can decide when they want to go and for how long. The question then arises: how different is this from an ordinary tour company? And if students can dip in and out of the projects, how important are the jobs they are doing?

Kevin Dynan, in charge of recruitment for the company (which would probably be called marketing elsewhere), points out reasonably enough that not all gappers are ready to make the 12-month commitment required by some organisations. 'With us, they go initially for three months, which is long enough to stick it out and also to make some contribution.' He says there is some in-country induction, and in areas where cultural faux pas are a real risk, 'we might have a chat with people before they go'.

He acknowledges that there are periods when no one is filling the posts, 'but in teaching, it's not necessarily a problem, as they're very much language assistants. Other projects are more about training - shadowing a doctor or working in a radio station'. And what's the difference between them and a holiday company? 'We're not interested in making huge amounts of profit.'

It is, in fact, extremely difficult to compare the costs of different organisations, because advertised prices often don't include airfares, insurance, pocket money or language and induction programmes and cover different periods of time. As a general rule, expeditions are more expensive than volunteering or work placements, and you are unlikely to get away with much under £3,000. Lavinia Bristol of Project Trust has attempted to calculate relative costs and concluded that Gap is the cheapest (with Project Trust next) and something called, improbably, Ski Le Gap is the most expensive.

Not least thanks to cheap flights and a better travelled population, the popularity of gap years looks set only to rise. Eighteen-year-olds are a tremendous resource and eager to be used, especially if that buys them a period in an exotic place.

Plugging the gap

Top 10 Tips

1. Plan at least a year ahead. Places on projects fill up months ahead.

2. Do your homework by using guidebooks such as Lonely Planet or the Rough Guide series. Taking a Gap Year by Susan Griffith, published by Vacation Work (www.vacationwork.co.uk includes a directory of useful organisations .